1. Field of the Invention
The present invention relates to exposure control devices for use in still cameras and the like, and more particularly to a shutter control device for automatically establishing exposure time in accordance with the level of scene illumination.
2. Description of the Prior Art
In many prior art shutter control devices which include timing circuits for establishing shutter speed, the circuit is energized by a battery contained in the camera. Such mechanisms usually include an electromagnetic transducer adapted to close the shutter after a period of time determined by the timing circuit. Although such arrangements are generally satisfactory, they rely on the availability of sufficient power from the battery. When the battery power is inadequate for operation of such systems, either the shutter operates at a fixed, predetermined speed or it remains open indefinitely, depending on the type of shutter control.
Photographic apparatus such as cameras that employ electric generators for energizing exposure control circuits have been described in the patent literature, such as in U.S. Pat. No. 3,598,030. However, we do not know of any commercial use of such apparatus, which commonly are shown with a spring-driven member to rapidly rotate the generator armature. One possible disadvantage which may have discouraged the use of such generators might be a tendency to cause camera vibration and noise during armature run-down. Since the energy conversion of such generators is inefficient, a substantial amount of energy must be stored in the spring, and this is generally done during manual film advance or by preliminary shutter release button movement. The greater the amount of energy needed to power the generator, the more physical effort is demanded from the operator.
In commonly assigned, co-pending U.S. Pat. application Ser. No. 816,776, entitled PIEZOELECTRIC CAMERA SHUTTER, filed in the names of L. F. Frank and J. K. Lee on July 18, 1977, a shutter control device was disclosed which employed a piezoelectric bimorph for providing electrical energy for powering an electronic exposure control circuit and a second bimorph for converting an electric control signal from the circuit to mechanical energy to terminate exposure. The shutter control device disclosed in that application represented a significant improvement over previously known mechanisms because piezoelectric devices are more reliable than battery sources and more efficient than conventional, non-battery powered sources known in the prior art for powering camera exposure control system. In fact piezoelectric devices have very little resistive loss, and may operate at electro-mechanical conversion efficiencies as high as about 50%. As motors, piezoelectric devices are extremely fast acting and therefore permit very accurate timing while operating at low energy levels. They may be latched in a selected position without draining power and have a theoretically infinite operational life.
The present invention is an improvement over the mechanism shown in aforementioned U.S. Patent application Ser. No. 816,776, wherein the functions of the generator bimorph and the motor bimorph are performed by a single bimorph, thus effecting a savings in elements and associated manufacturing costs.